The term "tabular grain" is employed to indicate a silver halide grain having an aspect ratio of at least 2, where "aspect ratio" is ECD/t, ECD being the equivalent circular diameter of the grain (the diameter of a circle having the same projected area as the grain) and t is the thickness of the grain.
The term "ultrathin tabular grain" is employed to indicate a tabular grain of a thickness less than 0.07 .mu.m.
The term "tabular grain emulsion" is employed to indicate an emulsion in which tabular grains account for at least 50 percent of total grain projected area.
The term "high chloride" or "high bromide" as applied to a grain or emulsion is employed to indicate that the grain or the grains of the emulsions contain at least 50 mole percent chloride or bromide, respectively, based on total silver present in the grain or the grains of the emulsion.
The term "{111} tabular grain" is employed to indicate an emulsion in which the parallel major faces of the tabular grain lie in {111} crystal planes.
The overwhelming majority of photographic applications currently employing high aspect ratio (e.g., &gt;8) tabular grain emulsions are served by those emulsions in which the tabular grains contain at least 50 mole percent bromide, based on total silver. Iodide in varying amounts occasionally ranging up the saturation level of iodide in the silver bromide crystal lattice (approximately 40 mole percent) are often incorporated into the tabular grains to enhance photographic sensitivity. Wilgus et al U.S. Pat. No. 4,434,226, Solberg et al U.S. Pat. No. 4,433,048 and Kofron et al U.S. Pat. No. 4,439,520 disclose representative high aspect ratio tabular grain silver bromide and iodobromide emulsions. (In mixed halide grains the halides are named in order of ascending concentrations.)
In precipitating thin tabular grain silver bromide and bromoiodide emulsions, it is recognized that the bromide ion concentration in solution at the stage of grain formation must be maintained within limits to achieve the desired tabularity of grains. As grain growth continues, the bromide ion concentration in solution becomes progressively less influential on the grain shape ultimately achieved. For example, Wilgus et al U.S. Pat. No. 4,434,226 teaches the precipitation of high aspect ratio tabular grain silver bromoiodide emulsions at bromide ion concentrations in the pBr range of from 0.6 to 1.6 during grain nucleation, with the pBr range being expanded to 0.6 to 2.2 during subsequent grain growth. Kofron et al U.S. Pat. No. 4,439,520 extends these teachings to the precipitation of high aspect ratio tabular grain silver bromide emulsions. pBr is defined as the negative log of the solution bromide ion concentration. Daubendiek et al U.S. Pat. No. 4,414,310 describes a process for the preparation of high aspect ratio silver bromoiodide emulsions under pBr conditions not exceeding the value of 1.64 during grain nucleation. According to Maskasky U.S. Pat. No. 4,713,320, in the preparation of high aspect ratio silver halide emulsions the useful pBr range during nucleation can be extended to a value of 2.4 when the precipitation of the tabular silver bromide or bromoiodide grains occurs in the presence of gelatino-peptizer containing less than 30 micromoles of methionine (e.g., oxidized gelatin) per gram.
High chloride ultrathin {111} tabular grain emulsions are disclosed in Maskasky U.S. Pat. No. 5,217,858, wherein triaminopyrimidine grain growth modifiers are used in the preparation thereof containing 4, 5 and 6 ring position amino substituents, with the 4 and 6 position substituents being hydroamino substituents. The term "hydroamino" designates an amino group containing at least one hydrogen substituent--i.e., a primary or secondary amino group. The triaminopyrimidine grain growth modifiers include both those in which the three amino groups are independent (e.g., 4,5,6-triaminopyrimidine) and those in which the 5 position amino group shares a substituent with 4 or 6 position amino group to produce a bicyclic compound (e.g., adenine, 8-azaadenine, or 4-amino-7,8-dihydro-pteridine). High chloride {111} tabular grains, unlike high bromide {111} tabular grains, cannot be formed or maintained in the absence of a grain growth modifier, but rather would take nontabular forms, since {100} crystal faces are more stable in high chloride grains. The high chloride ultrathin {111} tabular grain emulsions are prepared by a double jet process in which silver and halide ions are concurrently run into a dispersing medium containing the grain growth modifier. The first function of the grain growth modifier is to promote twinning while grain nucleation is occurring, so that ultrathin grains can form. Thereafter the same grain growth modifier or another conventional grain growth modifier can be used to stabilize the {111} major faces of the high chloride tabular grains.
The art has long recognized that distinctly different techniques are required for preparing high chloride {111} tabular grain emulsions and high bromide {111} tabular grain emulsions. For example, U.S. Pat. No. 5,217,858 does not disclose the processes of preparing high chloride ultrathin {111} tabular grain emulsions to be applicable to the preparation of high bromide ultrathin {111} tabular grain emulsions. Further, since at low pBr the {111} major faces of high bromide tabular grains have no tendency to revert to {100} crystal faces, the precipitation of high bromide {111} tabular grain emulsions generally has not required the addition of compounds comparable to the grain growth modifiers of U.S. Pat. No. 5,217,858. Daubendiek et al U.S. Pat. No. 4,914,014, Antoniades et al U.S. Pat. No. 5,250,403 and Zola et al EPO 0 362 699 illustrate the preparation of high bromide ultrathin {111} tabular grain emulsions wherein silver and bromide are introduced into a reaction vessel during growth of the high bromide tabular grains. None of such references, however, suggest the use of a grain growth modifier to prepare high bromide ultrathin {111} tabular grain emulsions.
Verbeeck EPO 0 503 700 discloses reduction of the coefficient of variation (COV) of high bromide high aspect ratio {111} tabular grain emulsions through the presence of an aminoazaindene, such as adenine, 4-amino-pyrazolopyrimidine and substitutional derivatives, prior to the precipitation of 50 percent of the silver. Double jet precipitation techniques are employed. The minimum disclosed thickness of a tabular grain population is 0.15 .mu.m.
Maskasky U.S. Pat. No. 5,411,851 discloses the preparation of high bromide ultrathin tabular grain emulsions wherein a triaminopyrimidine grain growth modifier of the type described in U.S. Pat. No. 5,217,858 is used during a grain growth process which comprises ripening of seed grains as opposed to growth of grains by addition of silver and halide in a double jet process. U.S. Pat. No. 5,411,851 discloses that pBr for such ripening grain growth process is optimally greater than 2.6, and states that the grain growth modifiers were ineffective in producing ultrathin {111} tabular grain emulsions during a double jet precipitation process. Ripening grain growth processes, however, are generally hard to reproducibly control and commercially scale-up.